Reciprocating air column sifter



Nov. 12, 1968 Filed Nov. 26, 1965 C. W. WARD ET AL RECIPROCATING AIR COLUMN SIF'TER 4 Sheets-Sheet l PI! HUN as 55 75 IN VEN TORS CHA R LES W. WARD JOSEPH P. LORBIECFKI ROBERT L. SCHURRER AT TOR N E Y Nov. 12, 1968 r; v WARD ET AL 3,410,400

RECIPROCATING AIR COLUMN SIFTER Filed Nov. 26, 1965 4 Sheets-Sheet 4 I I I III I \m Q I I I) I I q a I I k m m N COI N \g t0 I u v 1 Mi; w x o o I '12) I o N 4 gr I 3 Q //1/II N ///A i x I I :i

I I0 I l I I fi [0 O O r v N |I+ \O Q g N i m (0 B J I/ (O Q Q iii" I II I l (3\ r I n \Q P n I n I (q I I"""' I l i n I I Q I I INVENTORS CHARLES W,WAIRD JOSEPH R LORBIECKI b i ROBERT L.SCHURRER I.

ATTORNEY United States Patent 3,410,400 RECIPROCATING AIR COLUMN SIFTER Charles W. Ward, Milwaukee, Joseph P. Lorbiecki, New

Berlin, and Robert L. Schurrer, Milwaukee, Wis., as-

signors to Allen-Bradley Company, Milwaukee, Wis., a

corporation of Wisconsin Filed Nov. 26, 1965, Ser. No. 509,746 8 Claims. (Cl. 209-21) ABSTRACT OF THE DISCLOSURE A reciprocating air column sitter has a screen housing and a speaker housing mounted thereon. The screen housing has a cover with a speaker opening, A protective diaphragm covers the opening, and the speaker flange rests on the diaphragm. A cylindrical wall rests on the flange, and atop the wall is a top plate which has a tuning opening aligned with the speaker opening and surrounded by an upstanding rim. Threaded posts extend upwardly through the cover, diaphragm edge and flange, alongside the wall, and through the top, and are used to hold said elements together, and extend above the top near the rim. An elastic tuning diaphragm with a hoop larger than the rim is across the tuning opening, and a rigid guard screen above this diaphragm has arms which rest on the hoop and receive the ends of the posts. Nuts above the arms are used to secure the guard and tighten and loosen the diaphragm to vary resistance to movement of the speaker cone. The screen housing has a side access opening with a removable door, and horizontal ledges on its front and rear walls against which the edges of a screen lie. Slide members operable through the access opening have cam mountings on the front and rear walls and are adapted to clamp the screen against the ledges in a horizontal position. A tapper on the door is operable when the door is in place to periodically tap the screen. The screen housing has an inlet above and to the rear of the screen, an outlet below the screen, and a waste opening near the front of the screen. Photosensitive means are provided to read the flow of material through the screen and are coupled to the speaker to continuously vary the power applied to the speaker in response to the flow of material through the screen.

This invention relates to reciprocating air column sitters; and it resides more particularly in a sitter that is tuneable and otherwise controllable so that it is especially suitable for continuous, high-output industrial applications.

Reciprocating air column sitters are shown in US. Patent No. 3,045,817 to C. W. Ward and the co-pending application of Howard 0. Suhm et al., Ser. No. 473,949, filed July 22, 1965, now US. Patent No. 3,366,234. In general, such sitters comprise one or more screens and a sonic vibrator, usually a conventional dynamic loudspeaker. The speaker faces the screen or screens and is operated to establish a reciprocating air flow to aid in sitting material therethrough. Reciprocating air column sitters are highly etficient and useful in a wide variety of applications ranging from precision laboratory powder analysis to heavy duty, continuous run sifting applications.

To achieve maximum efliciency, a reciprocating air col umn sitter must be tuned. That is, the vibrator, screen and appurtenant elements form a mechanical spring-mass system that has a natural frequency of vibration. The vibrator is usually driven from an alternating current supply which also has a set frequency. If these frequencies are closely matched, the entire system is in resonance or tuned and will use minimum power for a given amount of work.

3,410,400 Patented Nov. 12, 1968 Tuning is especially necessary for industrial applications where power consumption and general etficiency are critical factors. Prior art approaches to tuning are not. however, entirely satisfactory for industrial sitters. In the aforementioned Ward patent, for example, it is intended that tuning be accomplished by varying the frequency of the alternating current supply. This atfords flexilibity, but requires the use of variable oscillators and other expen' sive equipment and also complicates operation.

In the aforementioned Suhm et al. application, tuning is accomplished by designing the sitter so that it has a natural mechanical frequency near 60 cycles per second and will operate at near maximum efiiciency when operated off a standard A-C supply. This approach to tuning is economical and eflicient, but may not afford the flexibility desirable for industrial applications. The elements of an industrial or commercial sitter are necessarily rather large and space and other factors govern their design to a large extent so that it is diflicult to establish the natural mechanical frequency of the system with real accuracy. Further, diaphragms and other elements are likely to become slack or otherwise be affected as the result of age, use and environmental changes such as humidity which cannot be controlled in a factory situation. Even the type of material being sifted and the output rate may have an effect on tuning. Thus, variability is important to allow a commercial sitter to be initially tuned. and kept in tune during long use under changing conditions.

Further, power and flow rate must be carefully controlled in a commercial sitter. Excessive or inadequate power supply to the vibrator, for example, can sharply reduce sifting efiiciency and result in improper separation or waste. It is highly preferable that such control be con-' tinuous and automatic to obviate the necessity for constant inspection and adjustment and to prevent substantial periods of inefiicient operation.

It is the general object of this invention to provide an improved reciprocating air column sitter especially, but not only, suitable for industrial applications.

It is one specific object of the invention to provide a sitter that is mechanically tuneable, and in which tuning can be accomplished simply and accurately.

It is another object of the invention to provide a sitter incorporating automatic, continuous control means for the sonic vibrator.

It is still another object of the invention to provide a sitter in which the inclination of a screen can be controlled to improve operating efficiency.

It is still another object of the invention to provide a sitter of modular nature whereby two or more sitter units can be stacked to provide for successive particle separations.

It is still another object of the invention to provide a sitter in which a screen can easily be inserted and removed.

It is still another object of the invention to provide a sitter having the foregoing features and advantages which is durable and long-wearing, has various safeguards to insure continuous eflicient operation, is easy to assemble, operate, inspect and repair, and is relatively simple and inexpensive to manufacture.

Other objects and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawings, forming a part hereof, in which there is shown, by way of illustration and not of limitation, a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a side view in elevation, partially broken away, of a reciprocating air column sitter formed according to the invention,

FIG. 2 is a rear view in elevation, with parts shown broken away and in cross section, of the sifter of FIG. 1,

FIG. 3 is an enlarged fragmentary view in cross section through the plane 33 shown in FIG. 2,

FIG. 4 is a fragmentary view in cross section through the plant 4-4 shown in FIG. 3,

FIG. is an enlarged fragmentary view in cross section through the plane 5-5 shown in FIG. 1, and

FIG. 6 is a schematic diagram of a power control circuit for the sifter of FIG. 1.

The sifter shown in the drawings is mounted on a frame designated generally by the reference numeral 1 which comprises four vertical corner posts 2 of square tubular cross section, angle side braces 3 extending between and secured to the tops and bottoms of the posts 2 on both sides of the frame 1, and suitable front and rear braces 4 secured to and extending between the posts 2. A control cabinet 5, seen in FIG. 1, is secured between the posts 2 at the front of the frame 1, to the left as seen in FIG. 1, and houses electrical control components for the sifter.

The sifter proper comprises a screen housing designated generally by the reference numeral 6 mounted on the frame 1 and a sonic vibrator housing designated generally by the reference numeral 7 that is mounted atop the screen housing 6. The screen housing 6 is formed of sheet metal and comprises a front wall 8, a rear wall 9, opposite side walls 10 and a cover or pour plate 11, which together define a sifting chamber. A screen 12 of any suitable mesh is disposed horizontally across such sifting chamber. Below the screen 12, a wall 13, seen in FIGS. 1 and 3, divides the screen housing 6 to form an outlet portion 14 terminating in a tubular outlet nozzle 15 near the rear of the sifter and a waste portion 16 terminating in a tubular nozzel 17 near the front of the sifter.

As can be seen most clearly in FIG. 3, the screen housing 6 is mounted between the upper side braces 3 by means of bolt, nut and collar assemblies 18 which extend between the braces 3 and side walls 10. The housing 6 is thus suspended on the frame 1 but is free to pivot forwardly and rearwardly. An adjusting arm 19, seen most clearly in FIG. 1, is mounted at its outer end on one corner post 2 and has an inner end alongside the corresponding side wall 10 that is provided with an elongated slot 20. A bolt 21 mounted on and extending from the wall 10 passes through the slot 20, and moves along the slot as the housing 6 is pivoted. A wing nut 22 is used to clamp the housing 6 in any desired pivotal position, A pointer 23 on the arm 19 and a scale 24 on the wall 10 indicate the pivotal position of the housing 6. Preferably, the slot 20 is long enough to allow the housing 6 to pivot about six degrees beyond vertical in a counterclockwise direction as seen in FIG. 3 and about three degrees beyond verical in a clockwise direction.

The screen 12 is rectangular and is soldered along its edges to a rectangular frame 25. The rear and side portions of the frame 25 are of rectangular cross section; but the front portion 26, as can be seen most clearly in FIG. 3, is generally C-shaped in cross section and, when the screen 12 is in place, overhangs an inwardly bent upper edge 27 of the wall 13. As can be seen in FIG. 3, the rear housing wall 9 is bent inwardly to define a rear screen ledge 28. As can be seen in FIG. 4, formed horizontal metal strips are affixed to the walls 10 to define side screen ledges 29. A relatively thick gasket is glued to the ledges 28 and 29.

When the screen 12 is in place, the rear and side portions of the frame 25 are tight against the gasket 30 and ledges 28 and 29. The screen 12 is held in this position by elongated front and rear slides 31 which are mounted at the front and rear of the housing 6 and bear, respectively, against the undersides of the front frame portion 26 and rear side of the frame 25.

The front slide 31 is shown clearly in FIG. 4 where it can be seen that the slide 31 is provided with laterally spaced generally inclined slots 32 provided at their lower ends with horizontal lock portions 33. Pin means 34 of any suitable type extend from the wall 13 and pass through the slots 32 to mount the slide 31 for movement inwardly and outwardly with respect to the housing 6. The outermost end of the slide 31, the end to the left as seen in FIG. 4 is bent rearwardly to define a manually engageable tab 35 by means of which the slide 31 can be moved.

The pin means 34 and slots 32 serve as a cam mounting means for the slide 31. When the slide 31 is in its innermost position, shown in full lines in FIG. 4, the pin means 34 are at the bottom or outer ends of the slots 32 and the upper surface of the slide 31 is tight against the frame portion 26 to hold its ends against the fronts of the side ledges 29. In this position, the pin means 34 are in the slot lock portions 33 so there is no substantial tendency for the slide 31 to move outwardly, to the left as seen in FIG. 4. When the slide 31 is pulled outwardly or to the left by means of the tab 35, it moves outwardly and downwardly to the position shown in broken lines in FIG. 4, freeing the frame portion 26.

The rear slide 31 is substantially identical to the front slide 31 and is mounted on pin means 36 extending from the rear wall 9. It is, however, mounted higher and bears primarily against the rear portion of the frame 25 to urge it against the rear ledge 28.

The left side wall 10 as seen in FIGS. 2 and 4 is provided with an access opening 37 through which the screen 12 may be inserted and removed and through which the tabs 35 of the slides 31 can be reached. The opening 37 is normally closed off by a door 38 secured to the side Wall 10 by three screws 39. A pair of arms 40 are pivotally connected at one end to opposite sides of the door 38 near its top, and at their other ends are pivotally connected to the front and rear walls 8 and 9. When the screws 39 are removed, the arms 40 allow the door to be moved from the normal position shown in full lines in FIG. 4 to the open position shown in broken lines wherein the opening 37 is exposed.

Mounted on the door 38 is a tapper assembly 41, preferably a solenoid, which has an inwardly extending reciprocating pin 42 that is at the level of and engageable with one side of the screen frame 25 when the door 38 is in place. During sifting, the tapper 41 is periodically actuated, for example every four seconds or so, to jar the screen 12 to prevent clogging.

Also mounted on the door 38 is the light element 43 of a photoelectric cell assembly which shines through an opening 44 in the door 38, the opening 44 being immediately under and near the forward edge of the screen 12 when the door 38 is in place. A receptor unit 45 is correspondingly located on the opposite side wall 10.

Below the screen 12, the rear wall 9 is provided with a relatively large, rectangular relief opening 46. A thin, elastic relief diaphragm 47, which is preferably formed of a polyurethane material a few thou-sandths of an inch in thickness, covers the opening 46, the edges of the diaphragm 47 being secured to a cardboard frame 48 which lies on the outer surface of the wall 9 about the opening 46. A metal frame 49 surrounds the opening 46 outwardly of the frame 48 and is secured to the wall 9 by screws 50, the frame 48 and diaphragm 47 being clamped in place between the frame 49 and wall 9.

Lying outwardly of and covering over the diaphragm 47 is a generally cylindrical rigid metal protective screen 51 of relatively large mesh. The ends of the screen 51 are welded or soldered to end plates 52 received between the sides of the frame 49 and fastened thereto by screws 53.

A barrier wall 54 is fastened to and extends between the side walls 10 above and toward the rear of the screen 12 and, with the rear wall 9 defines an inlet or entry passage for material to be sifted. In front of the barrier 54 is a sheet metal gate 55 provided with one or more vertical slots 56 which receive wing bolts 57 threadedly connected to the barrier 54. By means of the slots 56 and bolts 57, the gate 55 can be adjusted to vary the size of the discharge opening of the inlet passage formed by the wall 9 and barrier 54 and can be clamped in place in any desired adjusted position.

The cover plate 11 is removably fastened to horizontal flanges on the walls 8, 9 and 10 by means of bolts and wing nuts 58, gasket material 59 preferably being inserted therebetween. Near its rear edge, the cover plate 11 is provided with an upwardly extending inlet tube 60 which leads inwardly to the inlet passage formed by the walls 9 and 54. A relatively large, circular vibrator opening 61 is provided through the cover plate 11 substantially directly above the screen 12.

The vibrator housing 7 is mounted atop the cover plate 11 and, as can be seen most clearly in FIG. 3, includes a circular elastic dust protective diaphragm 62 similar to the diaphragm 47 which lies over the opening 61 and is secured about its periphery to a circular cardboard frame 63 which lies against the plate 11 about the opening 61. Above and substantially coextensive with the frame 63 is a rigid metal ring 64.

Above the ring 64 is a vibrator 65 which is a conventional dynamic speaker comprising a rigid basket 66, a come 67 and a magnet-voice coil assembly 68. As is usual, the basket 66 includes a mounting flange 69 beneath which is a compressible mounting gasket .70. The speaker 65 is disposed above the ring 64 with the cone 67 facing and in alignment with the vibrator opening 61. A thick ring 71 of gasket material is interposed between the gasket 70 and ring 64. If desired, two conventional speaker mounting gaskets similar to the gasket 70 can be used to make up the ring 71. The opening 61 is preferably somewhat smaller than the diameter of the speaker cone 67 to blank off the periphery of the cone 67 and prevent undesirable edge effects.

Above and surrounding the speaker 65 is a cylindrical sheet metal wall ,72, the upper and lower edges of which are covered by generally U-shaped sealing rings 73 formed of rubber or a suitable synthetic material. The lower edge of the wall 72, with its ring 73, is against the speaker mounting flange 69. Above and against the upper edge of the wall 72, and its ring 73, is a metal top plate 74 which is provided with a circular upstanding rim 75 which surrounds and defines a tuning opening 76.

Four circumferentially spaced threaded posts 77 extend through the cover plate 11, frame 63, ring 64, ring 71, gasket 70 and flange 69, pass alongside the wall 72 and extend through the top plate 74; and nuts 78 on the posts 77 above the top plate 74 and below the cover plate 11 serve to hold all these elements in place atop the screen housing 6. The posts 77 extend a substantial distance above the upper nuts 78 to receive tuning nuts as will be described.

Above the top plate ,74 and rim 75 is a tuning diaphragm assembly comprising a circular diaphragm 79 and a circular hoop 80. The diaphragm 79 is also formed of a suitable elastic material such as a polyurethane a few thousandths of an inch in thickness. The hoop 80 comprises a solid, rolled inner ring 81 and a split outer strap 82 between which the periphery of the diaphragm 79 is clamped, the outer strap 82 being tightened by means of a conventional threaded strap fastener 83 seen in FIG. 1. The hoop 80 is substantially larger in diameter than the rim 75 and is concentric therewith so that the diaphragm 79 rests on the rim 75. Vertical movement of the hoop 80 will, therefore, cause the diaphragm 79 to be loosened or tightened across the opening 76.

Above the diaphragm 79 and hoop 80 is a circular, rigid, domed protective metal screen 84 of relatively large mesh, the periphery of which is soldered or otherwise secured between an inner screen ring 85 and an outer screen ring 86, both somewhat smaller in diameter than the hOOp 80. Four rigid arms 87 have upturned portions at their inner ends secured to the inner ring 85 and extend radially outwardly under the outer ring 86 and over the hoop 80. At their outer ends, the arms87 are apertured and slidably receive the posts 77. Wing nuts 88 on the posts 77 above and below the arms 87 secure the screen 84 in place and also serve as means by which it and the rings 85 and 86 and arms 87 may be adjusted upwardly and downwardly on the posts 77. The arms 87 rest on the top of the hoop 80, and when they move downwardly the hoop 80 also moves downwardly to tighten the diaphragm 79. When the arms 80 moves upwardly, the hoop 80 will also move upwardly because of the elasticity of the diaphragm 79, and the diaphragm 79 will thus be loosened. The arms 87 are always, in normal use, against the hoop 80 and serve to hold it and the diaphragm 79 in place, this function being aided by the fact that the diaphragm 79 is pulled downwardly outside the rim in normal use and resists lateral movement.

In use, material to be sifted is inserted into the tube 60 and passes between the walls 9 and 54 and is deposited on top of the screen 12 near its rear edge. The speaker 65 is operated to cause reciprocating air movement through the screen 12 resulting from reciprocal movement of the speaker cone 67. This air movement enchances the sifting operation by constantly lifting and lowering the material on the screen 12 allowing smaller par ticles to move about in the mass of material until they can pass through the screen 12, and also aids in moving material across the screen 12 toward its front. The elastic relief diaphragm 47 allows for free air movement by reducing back pressure below the screen 12. Without the diaphragm 47, a great deal of additional space would have to be provided below the screen 12 and might increase the overall size of the sifter beyond reasonable limits.

Material sifted through the screen 12 passes through the outlet passage 14 and nozzle 15 for collection. Material passed over the screen, referred to herein as waste although it is usually not wasted and may be reprocessed, passes over the front of the screen 12 and out the passage 16 and nozzle 17 and can be collected for reprocessing or disposal. The tortuous path created by the front screen frame member 26 and turned wall portion 27 makes it virtually impossible for material to be transferred between the passages 14 and 16.

The tapper unit 41 is periodically actuated during sifting to insure against clogging of the screen 12, but the operation of the sifter is so efiicient that a tap every four seconds or so will usually be sufiicient, as opposed to other Sifters where constant blows are necessary. The protective diaphragm 62 prevents particles or dust from reaching the speaker 65 and impairing its operation.

The pivotal mounting of the door 38 provided by the arms 40 makes it easy to reach the screen 12 and also to expose the light 43 and tapper unit 41. The slides 31 al low for easy removal and insertion of the screen 12 while holding it securely in place during use.

Tuning of the sifter is accomplished by means of the diaphragm 79. Again, the entire sifter forms a mechanical system with a natural frequency of vibration. The speaker 65 is operated off a standard 60 cycle A-C supply, and it is desirable to bring the natural mechanical frequency of the system as close to this value as possible. A large speaker such as the speaker 65,. which may be a twelve or fifteen inch unit, has a relatively low natural frequency, about 25 c.p.s. The resistance presented by the system, including for example the diaphragms 47 and 62 and the air trapped in the sifter, tends to raise this frequency somewhat. If desired, the speaker cone can be spring mounted to prevent sag, and this will incidentally raise the natural frequency of the system still further. Final adjustment, however, is accomplished by tightening or loosening the diaphragm 79 which, in effect, raises or lowers the resistance behind the speaker cone 67 and correspondingly raises or lowers the natural mechanical frequency of the system.

To have maximum efficiency, the natural frequency of the system should be brought to within the range of 57- 63 c.p.s. The state of tuning can be tested by measuring the power required to operate the speaker 65. When the system is tuned as well as possible, power input will be reduced to a minimum. The design of the diaphragm 79 to properly effect tuning will necessarily depend to some degree on other aspects of the speaker system. In a system generally similar to that shown herein, however, a tuning diaphragm of a thickness on the order of 4 to 6 thousandths of an inch will usually allow for sufficient flexibility to provide adequate tuning.

Tuning flexibility is important at the outset and during the life of the sifter. That is, for example, the diaphragms 47 and 62 may become loose as the result of age or environmental conditions, and other factors may affect frequency. The variable tuning afforded by the construction herein is, therefore, of continuing importance.

The arms 87 and nuts 88 provide simple, readily accessible means to adjust the diaphragm 79 and also hold the screen 84 in place. If desired, however, the arms 80 could be mounted directly on the hoop 80.

In addition to tuning, other controls are provided by the construction herein. The gate 55, for example, controls the rate at which material is deposited on the screen 12. The pivotability of the unit on the frame 1 is important in that the screen 12, which is horizontal when the housing 6 is vertical, can be tilted forwardly or rearwardly to control the rate and manner in which material flows across the machine. For flufly material, for example, it is quite beneficial to have the screen 12 tilted upwardly from rear to front. For other materials, some appropriate downward tilt is advantageous.

A continuously variable level power modulating control is provided by the photo-sensitive system 43, 45 which reads the flow of material through the screen 12. A greater flow will of course result in less light reception by the unit 45. This reading is used to control the power applied to the speaker 65 by, as shown in FIG. 6, coupling the receptor 45 to the speaker 65 through an amplifier 89. By this means, the power applied to the speaker is made continuously dependent on flow. When flow increases, as when the proportion of fine to heavy material increases, power is automatically reduced, and power is increased when flow decreases. Power supplies and other details of the arrangement set forth have not been shown, but the structure and operation thereof will be understood by those skilled in the art.

An observation opening 91 is provided through the front wall 8 and is covered by a transparent window 92 which allows the operator to visually observe the action of material on the screen 12 for control and evaluation.

When dusty materials are being sifted, or after long use, a film of material may be deposited on the lens of the receptor 45 causing it to be blinded and resulting in a power shutoff. To guard against this possibility, it is desirable to provide means (not shown) such as compressed air means to continuously blow air across the receptor 45.

It is also desirable to prevent continued operation in the event of a blockage or material backup in the outlet passage 14. For this purpose, a hopper switch 90 of any suitable type is mounted adjacent the passage 14 and operates to shut off power to the speaker 65 when material backs up to a predetermined level.

The sifter herein is specifically constructed so that two or even more sifter units can be stacked to provide for successive sifting. The vibrator housing 7 and its appurtenant elements extend above the posts 2, but there is sufiicient space between the posts 2 at the bottom of the frame 1 to receive the same so that two sifter units can be stacked one atop the other. The braces 3 are at the ends of the posts 2 and outwardly of the housings 6 and 7 so that the upper braces 3 of one unit can easily be connected to the lower braces 3 of the unit above to form a unitary supporting framework. With two units stacked, a hose can be used to connect the inlet tube 60 of the lower unit to either the outlet nozzle 15 or waste nozzle 17 of the upper unit so that material passing therethrough can be resifted. The hopper switch 90 is especially advantageous when two units are stacked with the outlet 15 of the upper connected to the inlet 60 of the lower. Should the lower unit be unable to handle the output of the upper, there will be a backup in the connecting line and outlet 14 of the upper unit until it shuts off, operation being resumed when the lower unit has caught up.

Although a preferred embodiment of the invention has been shown and described herein, it will be obvious that modifications might be made without departure from the invention. Merely by way of example, some other reciprocating cone arrangement might be devised to replace the speaker 65. In view of the possible variations, the invention is not intended to be limited by the showing herein, or in any other way, except insofar as limitations appear specifically in the following claims.

We claim:

1. A reciprocating air column sifter comprising: a screen housing; a screen in the screen housing; a generally rigid, enclosed vibrator housing mounted on the screen housing and having a vibrator opening that faces the screen and a separate tuning opening therein; a vibrator in the vibrator housing that faces out through the vibrator opening and that includes a movable cone means that is adapted to be operated to establish a reciprocating air flow through the screen, the tuning and vibrator openings being effectively on opposite sides of the cone means; an elastic tuning diaphragm across the tuning opening; and adjustment means to tighten and loosen the tuning diaphragm to vary the resistance to movement of the cone means.

2. A sifter according to claim 1 wherein the vibrator housing is provided with an upstanding rim which defines the tuning opening; and the tuning diaphragm comprises an elastic diaphragm and a rigid hoop that is larger than and concentric with the rim; and the adjustment means operates against the hoop.

3. A sifter according to claim 2 wherein the adjustment means comprises a plurality of upwardly extending fixed threaded posts circumferentially spaced about and radially outwardly of the hoop, a plurality of rigid radial arms operatively associated with the hoop with ends slidably receiving the posts, and threaded nut means on the posts upwardly of the arms that are operable to move the arms downwardly.

4. A sifter according to claim 3 wherein there is a rigid guard screen covering the tuning diaphragm; and the arms are carried by the guard.

5. A sifter according to claim 4 wherein the screen housing includes a cover plate with a vibrator opening therethrough; and the vibrator housing includes an elastic protective diaphragm across the vibrator opening with an edge portion against the cover plate about the vibrator opening, and a speaker with a rigid basket including a mounting flange above the edge portion of the protective diaphragm, and a cylindrical wall atop the speaker mounting flange, and a top plate which includes the tuning opening and rim and that is atop the wall; and the posts extend through the cover plate, protective diaphragm and speaker mounting flange, pass alongside the wall and eX- tend through the top plate; and there are nut means on the posts below the cover plate and above the top plate to hold the cover plate, protective diaphragm, speaker, wall and top plate together.

6. A sifter according to claim 1 wherein the screen housing comprises a sifting chamber defined by front and rear walls, side walls and a cover plate which has the vibrator opening therethrough; and the screen is disposed generally horizontally across the sifting chamber; and there is an inlet above the screen through which material may be deposited on the top of the screen near the rear of the screen housing; and there is a waste opening leading from the screen near its front for material passed across the top of the screen; and there is an outlet opening below the screen for material sifted through the screen; and there is photosensitive means operative below 9 the screen near the front of the screen to measure flow of material through the screen; and there is means coupling the photosensitive means to the vibrator adapted to continuously vary power applied to the vibrator in response to fiow of material through the screen.

7. A sifter according to claim 1 wherein the screen housing comprises a sifting chamber defined by front and rear walls, side walls and a cover plate which has the vibrator opening therethrough; and the screen is disposed generally horizontally across the sifting chamber; and there is an inlet above the screen through which material may be deposited on the top of the screen near the rear of the screen housing; and there is a waste opening leading from the screen near its front for material passed across the top of the screen; and there is an outlet opening below the screen for material sifted through the screen; and wherein one side wall of the screen housing is provided with an access opening through which the screen may be inserted and removed; and there are horizontal ledge means inside the screen housing; and the screen has edge portions the upper surfaces of which are engageable with the ledges; and there are a pair of horizontal slides mounted on the front and rear walls of the screen housing below the screen, said slides being slidable into and out of the screen housing by means of manually engageable portions accessible through the access opening, said slides being engageable with the underside of the screen and having cam mountings on the screen housing so that movement of the slides inwardly causes the screen to be clamped against the ledge means; and there is a door adapted to cover the access opening, fastening means to removably secure the door in place over the access opening, and pivoted arm means operatively separate from the fastening means which allows the door to be swung away to expose the access opening; and there is tapper means mounted on the door that is adapted when the door is in place to periodically tap the screen to prevent it from clogging.

O vibrator above and facing the screen that is operable to establish a reciprocating air flow therethrough; photosensitive means operative below the screen near the front edge to read a flow of material therethrough; and means coupling the photosensitive means to the vibrator operable to continuously modulate power applied to the vibrator in response to flow of material through the screen.

References Cited UNITED STATES PATENTS 1,080,988 12/1913 McWilliams 250-215 X 1,342,663 6/1920 Cassca 181-31 1,967,223 7/1934 Bostwick 181-31 2,433,560 12/1947 Hurley 250-215 X 2,609,063 9/ 1952 Francis 55-277 X 2,903,135 9/1959 Dryg 209-236 3,045,817 7/ 1962 Ward 209-21 3,022,893 2/1962 Hasley 209-319 X 3,053,031 9/1962 Vedder 55-292 3,098,037 7/1963 Tonjes 209-260 3,151,699 10/1964 Peavey 1 81-31 3,154,465 10/ 1964 Degnen 55-277 1,363,207 12/1920 Rogers 209-319 X FOREIGN PATENTS 651,356 3/ 1951 Great Britain.

HARRY B. THORNTON, Primary Examiner.

R. L. HALPER, Assistant Examiner. 

